1. Field of the Invention
The present invention generally relates to a multi-mode electromagnetic target discrimination sensor system and method of operation thereof. More particularly, the present invention relates to a metal detection sensor system that operates in both time and frequency domain modes.
2. Description of the Related Art
Current state-of-the-art electromagnetic induction (EMI) metal detectors can detect the small amount of metal in plastic-cased land mines at shallow depths under a wide range of environmental and soil conditions. However, small and large metal non-mine objects (clutter) commonly found in the environment are a major complication in mine detection because they represent false targets. It has been estimated that metal clutter in the environment causes between 100 and 1000 false alarms for each real land mine. For time-efficient and cost-effective landmine clearing, the detected metal targets must be classified as to their threat potential: mine or clutter.
Two basic types of electromagnetic induction metal detector technologies are commonly used for mine detection: pulsed or time domain (TD) and continuous wave (CW) or frequency domain (FD). Vacuum tube metal detectors based on FD techniques were used during WWII for mine detection and some of the basic patents on this technology date to the 1940s. TD metal detectors started to appear in the mid 1950s for geological exploration of minerals. At present, both technologies are well developed and use modern computerized electronics for their operation. Currently, FD technology dominates the hobbyist market for metal detection, but in the area of mine detection, both technologies are available and have shown similar detection sensitivities for low metal mines. State-of-the-art hobbyist metal detectors appear to be more sophisticated than military metal detectors and use modern microprocessors for detection and discrimination. In fact, in one test, a hobbyist FD metal detector preformed better than some military-grade metal detectors. Each technology has its advantages and disadvantages.
Some advantages and disadvantages of TD and FD metal detection technologies are summarized in Table 1 and Table 2, respectively.
TABLE 1Advantages of TD and FD metal detection technologiesTD AdvantagesFD AdvantagesEasier to separate ground andHigh SNR for given conditionstarget responseFlexible antenna constructionHigh sensitivity for small targetsoptionsRelatively easy to make highMore power efficient for a givenbandwidth measurementstarget depth using resonance circuitsFaster spectral discriminationMore sensitive to antenna/targetmeasurementsorientation (an advantage anddisadvantage)Good metal target sensitivity inGood broadband noise rejectionmineralized soilTarget discrimination less affectedby mineralized soilEasy to classify simultaneous voidand metal signal
TABLE 2Disadvantages of TD and FD metal detection technologiesTD DisadvantagesFD DisadvantagesLower SNR for given conditionsSlower spectral discriminationmeasurementsMore power required for samePrimary field rejectionFD sensitivityTrade-off between bandwidthMore difficult to separate target andand sensitivityground responsePoor broadband noise rejectionLost sensitivity in mineralized andwithout averagingconductive soil and salt waterPoor discrimination capability inmineralized soilMotion sensitive
As the information above and below shows, TD and FD metal detection technologies have some overlapping capabilities in addition to their particular strengths and weaknesses. Fortunately, where one technology has a weakness the other technology has strength. For maximum sensitivity for a given total sensor power consumption, the FD technology possesses an advantage over the TD technology since a FD antenna can use resonance circuits to create a strong magnetic field for target detection. For target detection and discrimination, the TD technology can more easily separate mineralized and conductive soil and salt-water effects, compared to FD technology. In fact, some FD metal detectors have to ignore the signal (in-phase) from the mineralized soil so as not to overload the detector. Important information is then lost for target discrimination purposes. TD mode hobbyist metal detectors are used almost exclusively for underwater applications. Also, TD mode sensor excites the metal target with a continuous spectrum of frequencies based on the Fourier Transform of the TD sensor's transmitter current impulse compared to the discrete frequencies of the FD mode. For more detail, see also Carl V. Nelson, Toan Huynh and Charles Cooperman, “EMI Sensor with Both Time and Frequency Domain Technologies for Detection and Classification of Metal Objects,” SPIE, FL, 2001. Proceeding of SPIE, Detection and Remediation Technologies for Mines and Minelike Targets, Poster, Orlando, Fla., 16–20 Apr. 2001, the contents of which are incorporated herein by reference.
Extensive research has been conducted on FD and TD metal detector technology and algorithms for target classification. The basic pulsed-EMI technique used for metal detection can be described as follows. A current loop transmitter is placed in the vicinity of the buried metal target, and a steady current flows in the transmitter. The transmitter loop current is then turned off. According to Faraday's Law, the collapsing magnetic field induces an electromotive force (emf) in nearby conductors. This emf causes eddy currents to flow in the conductors. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field, the time rate-of-change of which is detected by a receiver coil located above the ground. The signal received by the receiver coil is a combination of the eddy currents from the metal target as well as the soil.
In the TD mode, the eddy current time decay response from a metal target can be expressed as                               V          ⁡                      (            t            )                          =                              δ            ⁡                          (              t              )                                -                                    ∑              i                        ⁢                          [                                                A                  i                                ⁢                exp                ⁢                                  {                                                            -                      t                                        /                                          τ                      i                                                        }                                            ]                                                          Equation        ⁢                                  ⁢                  (          1          )                    where t is time, V(t) is the induced voltage in the receiver coil, δ(t) is the delta function, Ai are target amplitude response coefficients, and τi are the target's time constants. Thus, the sensor response to a metal target is a sum of exponentials with a series of characteristic amplitudes, Ai, and time constants, τi. A similar expression can be written for a FD sensor. Equation (1) forms the theoretical basis of an EMI sensor's classification technique. If a target is shown to have a unique time decay or frequency response, a library of potential threat targets can be developed. When a metal target is encountered in the field, its time decay or frequency response can be compared to those in the library and, if a match is found, the target can be classified.
U.S. Pat. No. 5,387,900 discloses an electronic article surveillance (EAS) system with improved processing of antenna signals. The EAS system in which first and second received signals are independently front-end processed to produce third and fourth signals indicative of the absolute values of the first and second processed signals. The third and fourth signals are then combined and the combined signal passed to a tag evaluation processor for time and frequency domain processing for evaluating whether a tag is present in an interrogation zone. The front-end processing is carried out in such a way that interference signal content including shield interference is extracted without extracting tag signal content in the received signals over a period of time. In this way, the first and second transmitter antennas of the system can be driven with drive signals having a phase difference of other than 0.degree. or 180.degree. and the tag evaluation processing can be carried out during the entire period of the drive signals. The EAS uses a frequency domain signal in the transmitter circuit to excite a special response from the article surveillance tag placed on an object that is being protected from theft. The article surveillance tag is tuned to the frequencies that are transmitted.
U.S. Pat. No. 5,699,045 discloses an EAS system with cancellation of interference signals. The EAS system includes a signal generator for generating an interrogation signal (e.g., frequency domain) in an interrogation zone, an antenna which receives a signal present in the interrogation zone, and interference canceling circuitry for canceling interference components in the signal received by the antenna. The interference canceling circuitry includes a hybrid interference component canceling loop in which a digital interference estimate signal is formed and converted into an analog estimate signal, and the analog estimate signal is subtracted from an input analog signal. The resulting difference signal is processed with a hybrid automatic gain control loop. A digitized signal, formed from the resulting difference signal, is subjected to digital interference cancellation processes in addition to the hybrid interference component cancellation process. Each of the digital interference cancellation processing and the hybrid interference component cancellation loop entails performing a respective polyphase decomposition of a digital input signal, estimating a mean value of each of the resulting subsequences, and combining the estimated mean values to form an interference component estimate signal. An input sample window provided for a comb-filtering stage is adjusted in phase relative to the cycle of the interrogation signal to compensate for changes in phase of the marker signal to be detected.
U.S. Pat. No. 5,103,234 discloses an electronic article surveillance system. The magnetic article surveillance system utilizing microcomputer control and unique time domain and frequency domain information gathering channels whose information is processed by the microcomputer via preselected time domain and frequency domain criteria.
U.S. Pat. No. 4,859,991 discloses an electronic article surveillance system employing time domain and/or frequency domain analysis and computerized operation. The magnetic article surveillance system also utilizes microcomputer control and unique time domain and frequency domain information gathering channels whose information is processed by the microcomputer via preselected time domain and frequency domain criteria.
U.S. Patent Publication No. 2003/0016131 discloses a wide area metal detection (WAMD) system and method for security screening crowds. The Wide Area Metal Detection (WAMD) system and method for security screening a crowd of people is provided. The system comprises at least one Magnetic Field Generator (MFG), e.g., a Horizontal Magnetic Field Generator (HMFG) buried below a walking surface for generating a magnetic field, a plurality of magnetic field sensors located within the sensing area of the MFG for sensing a metallic object, based on eddy currents in the magnetic field, and a location indicator for indicating a location of an individual with the metallic object at a position corresponding to that of one of the plurality of magnetic field sensor that sensed the metallic object. At least one video camera is included for identifying the individual at the location indicated by the location indicator and tracking further movements of the individual.
U.S. Patent Publication No. 2003/0034778 discloses a portable metal detection and classification system. The metal detector system including a chassis for supporting electromagnetic sensor components above a medium such as soil or water. A transmitter coil and two receiver coils are attached to the chassis. A propulsion system is attached to the chassis between or adjacent to the receiver coils. The location of the propulsion system causes electromagnetic interference signals emanating from the propulsion system to be received at a nominally equal magnitude by each of the receiver coils.
U.S. Patent Publication No. 2003/0016131 discloses a steerable three-dimensional magnetic field sensor system for detection and classification of metal targets. The steerable electromagnetic induction (EMI) sensor system for measuring the magnetic polarizability tensor of a metal target. Instead of creating a vertical magnetic field from a horizontal loop transmitter configuration used by most prior art EMI metal detectors, the transmitter geometry of the sensor system's antenna is designed especially for creating multiple horizontal and vertical magnetic fields and for steering the same in all directions. The horizontal magnetic field (HMF) antenna has the potential advantage of a relatively uniform magnetic field over a large volume. A second potential advantage of the HMF antenna is that compared to a conventional loop antenna, the magnetic field intensity falls off slowly with distance from the plane of the antenna. Combining two HMF sensor systems creates a steerable two-dimensional magnetic field sensor. Combining the steerable HMF sensor with a vertical magnetic field antenna forms a three-dimensional steerable magnetic field sensor system.
U.S. Patent Publication No. 2003/0052684 discloses an electromagnetic target discriminator sensor system and method for detecting and identifying metal targets. The time-domain electromagnetic target discriminator (ETD) sensor system and method are provided capable of measuring a metal target's time decay response based on the physical parameters of the metal target and its environment and for identifying the metal target. The ETD sensor system includes a pulse transmitter connected to a receiver via a data acquisition and control system. The transmitter and receiver include coil configurations for placement in proximity to a visually obscured, e.g., buried, metal target (or underground void) for inducing eddy currents within the metal target. The ETD sensor system measures the eddy current time decay response of the metal target in order to perform target recognition and classification. The identification process entails comparing the metal target's (or, underground void or other object's) time decay response with a library of normalized object signatures, e.g., time decay responses and other characteristics.
Prior art metal detectors do not address the operation of a metal detector for both time multiplexed TD and FD modes with the antenna (transmitter and receiver coils) optimized for metal detection and target classification under all soil conditions. Multiple frequency FD mode metal detectors use a single transmitter and receiver coil configuration that just changes the transmitter resonance frequency without changing either the transmitter or receiver coil turns. TD mode metal detectors do not use variable antenna configurations to optimize the antenna to the target characteristics.